Nickel-base superalloy components of gas turbines are sometimes coated with aluminum and then heated to diffuse the aluminum into the surface of the article. The aluminum-rich surface is oxidized to produce an adherent aluminum oxide scale on the surface of the article. The aluminum oxide scale is an effective barrier against further oxidation and corrosion of the component in service.
The aluminum coating is typically applied by a vapor phase deposition process. In one such approach, aluminum fluoride gas is contacted to the component surface under conditions such that the compound decomposes to leave a layer of aluminum deposited on the surface. The aluminum diffuses into the surface during the deposition and any post-deposition heat treatment, producing the aluminum-enriched surface region.
It is sometimes the case in such deposition processes that a first portion of the surface of the article is to be left uncoated, and a second portion of the surface of the article is to be coated with aluminum. The uncoated portion may be required for any of several reasons, such as allowing attachment of other structure to the article by brazing. Any brazed joint between the aluminum coating and another structure is ordinarily brittle, so that the brazed joint must be made to the uncoated first portion.
In order to prevent deposition of aluminum from the aluminum-containing gas, the first (uncoated) portion of the surface of the article is physically covered with a mask. The mask prevents contact of the aluminum-containing gas to the first portion. Available maskants usually include sources of Ni.sup.+2 and Cr.sup.+3 ions in a binder complex with Al.sub.2 O.sub.3 particles. These maskants are intended to prevent the coating vapors from reaching the surface of the article, and to prevent depletion of the alloy components from the surface of the first portion of the surface.
The present inventors have observed that, after removal of the maskant from the first portion of the substrate surface, in some cases there are surface oxides on the first portion of the surface that impede subsequent brazing operations. It is believed that these surface oxides result from the chemical interaction of the maskant with the masked surface. The surface oxides cannot be readily removed in many cases, because their removal may cause damage to the aluminide coating. In addition, there may be surface depletion of elements from the substrate due to diffusional processes, altering the chemistry of the surface region of the substrate in an adverse manner.
These same types of problems arise in relation to other coatings as well, where masks are used. There is a need for an improved approach to the coating of a portion of an article surface with a coating such as an aluminide, where some of the surface must remain uncoated.